Studies on the pharmacokinetics of Citrovorum factor (CF) and Methotrexate (MTX). Preliminary studies indicate that simultaneous infusion of CF and MTX does not alter the excretion of MTX. This was studied at various plasma levels of MTX but only one dose of CF has been studied. The pharmacokinetics of 5-methyltetrahydrofolate (5-CH3THF), the major active metabolite of CF, was studied using 14C-labeled 5-CH3THF. It appeared to be handled in the body in much the same manner as CF in that plasma half-life, volume of distribution and renal excretion were similar for CF and 5-CH3THF. We have developed a high pressure liquid chromatography (HPLC) method for separating CF and its metabolites. Tritiated CF has recently become available and we are now prepared to describe the physiologic disposition of racemic (DL) CF as well as the disposition of the active (L) isomer. Studies of an alternative method for rescuing from MTX toxicity. We have developed a model system using the L-1210 mouse tumor and rescue with either CF or a thymidine/purine (TdR/PU) combination after high dose MTX therapy. Initial findings are that the increased therapeutic index obtained using MTX followed by rescue compared to optimal doses of MTX alone is similar with the two different rescue protocols. This suggests that the mechanism for the increased therapeutic index does not involve an effect of CF on the pharmacokinetics of MTX and that a biochemical difference between normal cells and tumor cells with respect to folate utilization is not involved. A cell kinetic basis for the enhanced therapeutic index obtained with high dose MTX followed by rescue is being studied.